So, in my
Automatons thread I mentioned capturing of souls to make the aforementioned automatons work via means of auxillarite crystals.
I would like to expand upon that concept here.
When a creature is killed, its soul will be released and ascend to...Armok knows where (which is funny, all things considered) and may come back to this world as already in-game ghosts. Certain creatures (notably clowns) have a voracious appetite for souls, and will "eat" them (this may have various effects, but generally speaking it will increase their power).
Souls posses any skills that a creature they belang to learned, but
this only applies to non-physical skills and attributes (so strength, agility, toughness, endurance etc.-related stuff is lost, since that is body-related and body-exclusive), should they be able to make use of them again.
Souls will try to escape the mortal plane, but may still haunt their bodies (or what was left of them, should there be any remains) for a short time (~half a day, Adventurer mode-wise)
Souls can also be forced to stay on the mortal plane by the use of mysterious crystals known as
Auxillarite. Those precious gems come in two varieties,
Blue and
Red and are affected by item qualities, which determines their "capacity".
Blue auxillarite crystals are rarer than
Red auxillarite crystals, but souls that are captured inside them can be released and potentially even given new bodies, should anything happen to the previous one.
Red auxillarite crystals are a more common variety, but any soul captured within can't be released, but instead will be continually transformed into a magical energy that can be used to...Many fun things, like magic, powering automata, feed your soul eaters and whatnot. The skills, and later stats, of a soul will be slowly drained ("experience" in a given skill/stat will start to decrease until it reaches 0) until all of them will "expire" and the soul will vanish, leaving an empty crystal.
Souls have different power based on their soul skills and attributes - this is where crystals' size comes in - while all crystals can hold any (1) soul, the crystal might shatter when a powerful soul is placed in a low-quality/weak crystal.
Crystals can be cut, which will increase their quality, thus making it possible to store better souls within safely.
Both varieties can also be found in the Circus, in fair amounts. Beware of any residents that might not necessarily enjoy their stuff being stolen.
In order to capture a soul (in Adv. mode, at least), get a crystal (how you're supposed to do that in Adventurer mode until labor skills are in is beyond me, but whatever), and find a (fresh-ish) corpse, or place of otherwise death. Via advanced interaction, you will be able to select a soul, much like when necromancing, but only one at a time. (souls will be essentially ghosts in that sense they're unharmable, bodiless creatures)
After a fitting announcement, the "=Red/Blue auxillarite crystal=" will turn into "=Charged red/blue auxillarite crystal=" and its description (or even contents for that matter) will show you information about a soul being captured, and you will be able to view its skills/stats/stuff.
I hope I explained it the best I can. The concept can be expanded upon a big deal, and we all want DF to be as diverse and epic as it can get.